0 The Platinum-Group Elements: 70 “Admirably Adapted” 5 75 for Science and Industry 10 80 James M. Brenan* DOI: 10.2113/ GSELEMENTS .4.4.227 15 85 he platinum-group elements (PGE) tend to exist in the metallic state or “Platinum is a most valuable bond with sulfur or other Group V ligands, and often occur as trace metal; as it is not oxidisable, Taccessory minerals in rocks. Combined with three isotopic systems that nor fusible under common contain the PGE, these elements afford a unique view of early solar system 20 circumstances, and only 90 evolution, planet formation and differentiation, and biogeochemical cycling. with difficulty combinable Initial purification of the PGE was accomplished in the late 1700s, at which with sulphur, and not acted time their unique properties, including high melting point, chemical inertness, upon by common acids, it is 25 and ability to catalyze chemical reactions, became apparent. This led to admirably adapted for the 95 enormous industrial demand, most notably for fuel production and engine uses of the philosophical emission control, which combined with scarcity in crustal rocks, has made chemist.” Elements of the PGE a highly valued commodity. Chemical Philosophy, 30 100 KEYWORDS : platinum-group elements, siderophile, chalcophile, alloy, sulfide, catalysis BY HUMPHREY DAVY INTRODUCTION the PGE fit into this picture? Most significantly, many of the chemical properties of the PGE that are of industrial 35 On 23 January 1817, Sir Humphry Davy delivered a lecture 105 to the Royal Society of London describing the results of his interest also result in distinct geochemical behavior, afford - recent experiments on the combustion of gas mixtures. Davy ing a unique view of early solar system evolution and reported “the discovery of a new and curious series of phe - planet formation and differentiation. The PGE, along with nomena,” in which mixtures of highly explosive coal gas rhenium and gold, are grouped together as the highly siderophile elements, which are defined by their extreme 40 and air were found to cause a heated wire of platinum to 110 “become ignited nearly to whiteness.” Once the glowing of partitioning into the metallic, relative to the oxide, phase. the wire was extinguished, it was found that the gas mixture The PGE are highly refractory, as gauged by their high melt - was no longer inflammable. Davy reported that “I tried to ing and condensation temperatures ( FIG . 2 ), and were there - produce these phenomena with various metals; but I have fore relatively concentrated in the feedstock for the terrestrial planets (Palme 2008 this issue). However, because of an 45 succeeded only with platinum and palladium; with copper, 115 silver, iron, gold and zinc, the effect is not produced.” What aversion to bonding with oxygen, the PGE are now scarce Davy had discovered was heterogeneous catalytic oxidation. in the crusts of differentiated planets and, instead, are con - Davy immediately recognized the practical application of centrated in the core-forming Fe–Ni alloy. As discussed in his discovery and modified the safe-lamp he had designed the articles by Palme (2008) and Lorand et al. (2008 this issue), this tendency to exist in the metallic state (termed 50 for coal miners to include a Pt wire around the lamp flame 120 (FIG . 1 ), thus limiting the amount of combustible gas available siderophile) or bond with sulfur and other group VA and to the flame and reducing the chance of explosion. VIA ligands (termed chalcophile) has thus provided geo - chemists with a sensitive tracer of differentiation processes Fast forward to the year 2007, and the impact of Davy’s that involved metal or sulfide phases. As an invaluable bonus, original discovery becomes apparent: ~9 million ounces of three radioisotope decay systems include the PGE as parent 55 125 Pt and Pd (worth ~US$7 billion) were produced for the het - (107 Pd– 107 Ag), daughter ( 187 Re– 187 Os) , or both ( 190 Pt– 186 Os). erogeneous catalytic oxidation of automobile exhaust gas. These systems have not only provided chronological infor - This is in addition to the amounts produced for jewelry and mation, but differences in parent-daughter fractionation other industrial processes (mostly catalysis related) involv - have produced distinct isotopic tracers of a variety of geo - ing one or more of the platinum-group metals (PGE: Os, Ir, chemical processes (Carlson et al. 2008 this issue). 60 Ru, Rh, Pt, Pd), totaling some 7 million ounces, worth 130 ~US$10 billion 1. How does an issue of Elements devoted to Owing to their scarcity, but enormous industrial demand, there is significant interest in understanding the rare occur - rence of mineable PGE concentrations in the highly depleted crust. In this context, Mungall and Naldrett (2008 this * Department of Geology, Earth Science Centre, University of Toronto 65 22 Russell St, Toronto, Ontario, Canada issue) emphasize the chalcophile behavior of the PGE and 135 E-mail: [email protected] the massive extraction capacity of magmatic sulfide liquids. 1 2007 PGE production statistics obtained from Johnson Matthey As a resident of London in the 1800s, Humphry Davy would market data tables (www.platinum.matthey.com/publications/). Total PGE values are calculated from metal prices averaged for have most certainly been subject to the poor air quality that the 12-month period. typified this time. Today, it’s not the burning of coal, but E LEMENTS , V OL . 4, PP . 227–232 227 AUGUST 2008 0 automobile exhaust that degrades the air quality of many DISCOVERY AND PURIFICATION OF THE PGE 70 urban centres. Autocatalysts have significantly reduced this In 1741, the metallurgist Charles Wood returned to problem (see below), but the PGE they contain (and those England with samples of platinum acquired in Jamaica, from other industrial sources) are released to the environment, which likely came from the placer deposits of the Choco and some of these PGE are bioavailable. As Rauch and district of Colombia 2. This material was given to Dr. William 5 Morrison (2008 this issue) describe, this is a matter of some 75 Brownrigg, whose description was read to the Royal Society concern, as the risks of PGE emissions to human health and of London by William Watson (Watson and Brownrigg 1750). the environment have not been accurately assessed. There In this description, the samples are referred to as “platina,” is a clear call for more intense research in this area. which is derived from the Spanish for “little silver” and alludes to their appearance ( FIG . 3 ). William Lewis per - 10 80 formed an exhaustive battery of tests on this newly found material and concluded that “it follows, that platina is not, as some believe, gold naturally debased by the admixture of some other metallic body, but a metal of the peculiar kind, essentially different from all others” (Lewis 1757). 15 85 One important discovery made by Lewis is that platina is soluble in aqua regia, “excepting a little blackish matter” (Lewis 1754), which suggested the presence of impurities in the raw sample. Subsequent work on this residue led to the 20 discoveries of iridium and osmium by Smithson Tennant, 90 reported to the Royal Society of London on 21 June 1804. In the case of iridium, Tennant writes, “I should incline to call this metal Iridium [from iris , Greek for rainbow], from the striking variety of colours which it gives, while dissolving 25 in marine acid [HCl]” (Tennant 1804). Tennant notes that 95 during one of the osmium purification steps, “a pungent and peculiar smell is immediately perceived.” He discovered that this smell “arises from the extrication of a very volatile metallic oxide,” providing the basis for the name Osmium , 30 which is derived from osme , Greek for smell. That smell, of 100 course, originates from the highly toxic osmium tetroxide. Three days later, Tennant’s colleague William Wollaston reported the discovery of rhodium (Rh), which was purified 35 from the solution remaining after precipitating platinum by 105 treatment with sal ammoniac (NH 4Cl). He indicated that this new element should be “distinguished by the name Rhodium [from rhodon , Greek for rose], from the rose-colour of a dilute solution of the salts containing it” (Wollaston 40 1804). The following year, Wollaston (1805) reported the 110 discovery of palladium, which was purified from the aqua regia solution used to dissolve raw platina. In his report to the Royal Society of London, Wollaston proposed the name FIGURE 1 Detail of the Davy Safe-Lamp (a) employed in British coal palladium, “from the planet that had been discovered mines, with the Pt wire modification (j and h) used to cat - nearly at the same time by Dr. Olbers,” referring to the dis - 45 alyze methane oxidation (after Davy 1817). REPRINTED WITH PERMISSION OF 115 THE ROYAL SOCIETY OF LONDON . covery of the Pallas asteroid (originally considered to be a planet) by the German astronomer Heinrich Olbers on March 28, 1802. Records from his monetary accounts (Chaldecott 1979) reveal that Wollaston garnered signifi - 50 cant income from a method he developed for platinum 120 purification from raw platina, which he only made public in the Bakerian Lecture to the Royal Society (Wollaston 182 9) shortly before his death in 1828. This income helped to furnish the endowment for the Wollaston Medal 55 awarded by the Geological Society of London ( FIG . 4 ). The 125 medal was originally cast in gold, but from 1846 to 1860 it was cast in palladium extracted from Brazilian gold donated by noted assayer and metallurgist P. N. Johnson. Ruthenium (Ru) was the last of the PGE to be discovered, 60 and was isolated from the insoluble residue after aqua regia 130 digestion of platinum ore from the Urals.